MOSCOW—The Kremlin criticized the U.S. decision to arm Syrian opposition fighters and said Washington's evidence that the Syrian regime is using chemical weapons was unconvincing, but said Friday that Moscow is "not yet" discussing its plans to deliver of air-defense missiles to the regime.

President Barack Obama on Thursday authorized the U.S. to arm fighters against the Syrian President Bashar al-Assad's regime, reversing a policy of giving only nonlethal support to the country's opposition in the two-year-old...